1. Field of the Invention
The present invention relates to a spiral spring.
2. Background Art
Spiral springs are widely used in various applications such as a valve timing adjuster for rotationally driving a cam shaft by rotative power inputted from the crankshaft in an internal combustion engine.
The valve timing adjuster includes a housing operatively connected to the crankshaft and a vane rotor operatively connected to the cam shaft, the internal space of the housing is divided into a retard chamber and an advance chamber by vanes in the vane rotor, and supplying hydraulic oil to one of the retard chamber and the advance chamber and discharging hydraulic oil from the other make it possible to change the rotational phase of the vane rotor relative to the housing.
The valve timing adjuster further includes a spiral spring for enhancing the startability of an internal combustion engine by retaining the rotational phase of the vane rotor relative to the housing at an intermediate phase between the most retarded position and the most advanced position.
The spiral spring is interposed between the housing and the vane rotor so as to he capable of biasing the vane rotor toward the intermediate phase on the advance side when the vane rotor is placed more toward the retard side than the intermediate phase is, and thereby the rotational movement of the vane rotor at the start of the internal combustion engine is retained at the intermediate phase so that the startability of the internal combustion engine can be enhanced.
The spiral spring is a member obtained by spirally winding an elongated wire rod in approximately the same plane and arrives at an elasticity retaining state by the relative circumferential movement of the inner end and the outer end in the diameter-reducing direction.
FIGS. 4A to 4C show plan views of a conventional spiral spring.
FIGS. 4A to 4C show a free length state, an initial torque generating state (a state in which the spiral spring is elastically deformed in the diameter-reducing direction from the free length state so as to generate a predetermined initial torque), and a maximum torque generating state (a state in which the spiral spring is elastically deformed in the diameter-reducing direction from the initial torque generating state so as to generate the maximum torque) of the spiral spring, respectively.
As shown in FIG. 4A, in a free length state, a conventional spiral spring is configured such that the radius of curvature is increased at an approximately constant rate from the inner end that is located radially inside toward the outer end that is located radially outside (the radius is increased at an approximately constant rate).
As shown in FIGS. 4B and 4C, when a conventional spiral spring having this configuration is brought into an elasticity retaining state such as the initial torque generating state or the maximum torque generating state, all coil parts ranging from the first coil part in the innermost place to the n-th coil part in the outermost place in the radial direction (the third coil part in the configuration depicted in the drawings) come into contact with other radially adjacent coil parts only in one location that is approximately the same position with respect to the circumferential direction (hereinafter referred to as a first circumferential position).
That is, in the aforementioned conventional spiral spring, the entire areas in the circumferential direction of all coil parts except in the first circumferential position are not subjected to frictional contact with other coil parts that are radially adjacent, and thus are areas where elastic deformation can freely occur. Accordingly, there is a problem in that the overall natural frequency of the spiral spring is low.
When such a conventional spiral spring is used in, for example, a valve timing adjuster disclosed in patent literature 1, resonance occurs as the output rotational speed of an internal combustion engine increases and the frequency of vibrations added to the spiral spring nears the natural frequency, thus imposing a large burden on the spiral spring.
Moreover, the coil part located in the middle with respect to the radial direction comes into contact with and is compressed by the coil part that is located radially inside and the coil part that is located radially outside in the first circumferential position, thus stress is concentrated on the first circumferential position at the time of elastic deformation movement, and there is a possibility of this part being damaged.